1. Field of the Invention
This invention relates to thermal recording heads used in thermal recording, and more particularly relates to a semiconductor device useful for improving the recording density of the recording section of a thermal recording system of the line printer type which uses an array of many heat generator elements. It also relates to a thermal recording head in which such a semiconductor device is incorporated to effect thermal recording with a high recording density.
A thermal recording system is now widely employed in the recording section of various kinds of recorders as a recording means permitting easy maintenance. Both a high recording speed and a high recording density are now required for such a thermal recording system.
In order to increase the recording speed of a thermal head for use in thermal recording system of the line printer type, it is effective to cause simultaneous energization of a number of selected heat generator elements. To satisfy the above requirements, a thermal head of the line printer type has recently been developed and used in which semiconductor devices each thereof including a plurality of transistors and a shift register for energizing the individual heat generator elements are combined with the heat generator elements. Such a thermal head will be called hereinafter a thermal head of the shift register type.
2. Description of the Prior Art
FIG. 1 shows an exemplary form of electrical connection of such a thermal head. In FIG. 1, the thermal head, which is generally designated by reference numeral 100, includes heat generator elements 101 and semiconductor devices 102a to 102p. Reference numerals 103 and 104 designate terminals for connection to the plus and minus power supply terminals, respectively, of a power source 1 provided for energizing the heat generator elements 101, and reference numerals 105 and 106 designate terminals for connection to the plus and minus power supply terminals, respectively, of a power source 2 provided for driving the semiconductor devices 102a to 102p.
After these power sources 1 and 2 have been connected to the terminals 103, 104 and 105, 106, respectively, when driving voltages are applied to terminals 107a to 107d, 108a to 108d, 109 and 110 at the required timing, the heat generator elements 101 can be selectively energized as desired in accordance with a picture signal.
FIG. 2 shows the electrical connection and structure of the semiconductor device 102d taken, by way of example, from among the semiconductor devices 102a to 102p shown in FIG. 1. Referring to FIG. 2, the semiconductor device 102d includes a plurality of transistors 112 connected in series with the heat generator elements 101 and the power source 1 for energizing the heat generator elements 101, a latch circuit 113 temporarily storing a picture signal which turns on-off these transistors 112, and a shift register circuit 114 for applying the picture signal to the latch circuit 113.
In the thermal head of the shift register type illustrated in FIGS. 1 and 2, the individual heat generator elements 101 are energized in a manner as described hereunder.
Referring again to FIG. 1, a picture signal is applied to the terminal 107a while applying a clock signal to the terminal 109. After transferring the picture signal between the shift registers of the respective semiconductor devices 102d to 102a in the order of the semiconductor devices 102d.fwdarw.102c.fwdarw.102b.fwdarw.102a through U-connections 111, a strobe signal is applied to the terminal 110 to apply the picture signal, which has been transferred and stored in the respective shift registers, to the latch circuits 113. Then, when an enable pulse is applied to the terminal 108a, the transistors 112 connected to the heat generator elements 101, which are selected to be energized among those connected to the semiconductor devices 102a to 102d, are turned on according to the picture signal stored in the latch circuits 113. Then, through each of the heat generator elements 101 to be energized, a current flows through a path from the plus power supply terminal of the power source 1.fwdarw. a heat generator element 101.fwdarw. a conductive transistor 112 to the minus power supply terminal of the power source 1 only during the period of application of the enable pulse.
It is well known that, through such heat generator elements energization as described above, a thermal head of the line printer type having many heat generator elements can effect high speed thermal recording. Therefore, any further detailed description of the heat generator elements energization is unnecessary.
The manner of energization of the heat generator elements of a thermal head of the shift register type as described above differs depending on the internal circuit structure of the semiconductor devices, and the configuration of the electrical connections shown in FIG. 1 is modified to have various forms taking into consideration the factors including the condition of using the thermal head in addition to the circuit function of the semiconductor devices. Irrespective of such various configurations of the electrical connections, the thermal head of the above described type is featured in that the circuit of each of the semiconductor devices for energizing the heat generator elements includes at least a plurality of transistors and a shift register circuit.
It will be seen from the above description that the thermal head of the shift register type has the circuit structure most useful for increasing the speed of thermal recording. However, it is in no way easy to manufacture a thermal head capable of thermal recording with a high recording density such as 16 dots/mm which has been attained by an electrostatic recording system.
The principal reasons therefor are as follows:
(1) It is technically difficult to provide electrical connections between the individual electrodes of the heat generator elements arrayed in line with a high density and the terminals of the heat generator elements energizing transistors in the semiconductor devices; and
(2) it is also technically difficult to form the multilayer interconnections exemplified in FIG. 1 which are required for the connection of the terminals of the semiconductor devices other than those connected to the individual electrodes of the heat generator elements.
Besides the two reasons described above, it is also an important problem to make heat generator elements which are arranged in line with a high recording density such as 16 dots/mm and yet are not destroyed by the application of energy thereto which energy is required for the formation of a color picture. It is not too much to say that a thermal recording thermal head of the line printer type capable of thermal recording with a recording density of 16 dots/mm has not yet been sold in the market for the reasons (1) and (2) described above.
Further, a thermal head having an external appearance as shown in FIG. 3 is preferred when such a high recording density thermal head is used in the recording section.
In FIG. 3, reference numerals 3, 4 and 5 designate a base plate supporting the heat generator elements of the thermal head, a thermosensitive recording paper, and a recording paper feed roller, respectively. Referring to FIG. 3, the thermosensitive recording paper 4 is pressed by the recording paper feed roller 5 against an array of the heat generator elements (not shown) arranged in line on the base plate 3 and is fed in the direction of the arrow, and the thermal recording thereon is effected by the selective energization of the heat generator elements array. A cover 3a covers a portion of the base plate 3 supporting the heat generator elements, and, beneath this cover 3a, a plurality of semiconductor devices having a circuit function as described with reference to FIG. 2 are arranged in a single line or plural lines substantially in parallel with the array of the heat generator elements.
The thermal head having an external appearance as shown in FIG. 3, in which the plural semiconductor devices are arranged substantially in parallel with the heat generator element array only on one side of the heat generator element array and the cover 3a is provided for the purpose of mechanical protection of the semiconductor devices, is advantageous in that the portion of the recording paper 4 under recording is readily visible and the recording section can be made compact. However, if such an external appearance is taken up as one of the requirements for the thermal head, it becomes all the more difficult to manufacture a thermal head capable of thermal recording with a high recording density.
With the structure of a thermal head having semiconductor devices arranged on both sides of the heat generator element array as shown in FIG. 4, it has been possible to develop a thermal head capable of thermal recording with a high recording density of 16 dots/mm. However, in the case of thermal heads which have an external appearance shown in FIG. 3 and which are currently in production, the highest recording density is only 10 dots/mm.
In addition, as an example of thermal heads having an external appearance as shown in FIG. 4, a thermal head having a recording density of 12 dots/mm is known in which semiconductor devices of the shift register type are arranged under one of the mechanical protecting covers 3a and a diode array is arranged under the other of the mechanical protecting covers 3a.
It is an object of the present invention to provide the structure of a thermal head employing semiconductor devices useful for constructing a line printer type thermal head of the shift register type having the external appearance shown in FIG. 3 and capable of thermal recording with a high recording density of 16 dots/mm.
Another object of the present invention is to manufacture such semiconductor devices and thermal head as described above at a low cost.